Electrical Machine Design MCQ

1. The pole shoes of a dc machines

(a) are always laminated.
(b) are never laminated.
(c) are sometimes laminated.
(d) are partially laminated. [A.M.I.E Sec B. Electrical Design 2006]

2. Poles of dc machines are often laminated to

(a) reduce pulsation loss.
(b) reduce armature reaction.
(c) reduce iron weight.
(d) dissipate more heat. [A.M.I.E Sec B. Elec. Design 1993]

3. The number of poles for a dc machine is determined by

(a) frequency of flux reversal.
(b) flash-over between brushes.
(c) commutator length.
(d) overall size of the machine.
(e) all of the above.

4. In a dc machine, with the increase in number of poles all of the following increases except

(a) iron losses.
(b) labor charges.
(c) overall size of the machine.
(d) frequency of flux reversal.
(e) tendency of flash over.

5. As the number of poles in a dc machine increases the chances of flashover increases because

(a) the voltage between the segments increases.
(b) the voltage between segments decreases.
(c) the length of the commutator decreases.
(d) effect of armature reaction increases. [A.M.I.E Sec B. 2005]

6. Suitable value of number of poles for a 250 kW, 600 rpm dc motor is

(a) 2
(b) 4
(c) 6
(d) 8

7. Use of large core length in a dc machine

(a) reduces the cost of the machine.
(b) results in bad commutation.
(c) facilitates ventilation.
(d) both (a) and (b).

8. In a large size dc machine the conductors are of ……… in cross section.

(a) circular
(b) rectangular
(c) triangular
(d) none of these

9. In a dc machine the diameter of an armature is governed by

(a) pole pitch,
(b) peripheral speed,
(c) core length.
(d) both (a) and (b).

10. In a dc machine the peripheral speed should not normally exceed

(a) 15 m/s
(b) 30 m/s
(c) 50 m/s
(d) 60 m/s

11. In dc machines the width of ventilating ducts usually varies from

(a) 4 to 5 mm.
(b) 8 to 10 mm.
(c) 12 to 15 mm.
(d) 2 to 3 mm.

12. Air gap at the pole tips of a dc machine is kept more than that at the centre of the pole mainly to reduce

(a) reactance voltage.
(b) effect of armature reaction.
(c) losses in armature core.
(d) noise of machine. [A.M.I.E. Elec. Design 1994]

13. In dc machines air gap is ……….. in comparison to that in ac machines.

(a) very short
(b) short
(c) longer
(d) any of these

14. DC machines designed with a large value of air gap length

(a) provides better ventilation.
(b) reduces the pulsation losses.
(c) reduces the distortion effect.
(d) all of the above.

15. The air gap provided in dc machines of smaller diameters and provided with lesser number of poles is comparatively

(a) smaller.
(b) longer.
(c) any of these.

16. DC machines provided with a large value of air-gap length

(a) provides comparatively quiet operation.
(b) provides better ventilation.
(c) provides poor ventilation.
(d) both (a) and (b).

17. In a dc machine, for the same values of φ, Z and N; which one of the following statements is correct?

(a) Armature emf is more with wave winding than with lap winding.
(b) Armature emf is less with wave winding than with lap winding.
(c) Armature emf depends on whether the machine is running as a motor or a generator.
(d) Armature emf is the same as long as the flux density in the air gap remains the same.

18. Number of parallel paths in a dc machine with duplex wave winding is

(a) 2
(b) 4
(c) 8
(d) 16

19. The number of parallel paths for a 4-pole duplex lap winding will be

(a) 2
(b) 4
(c) 6
(d) 8 [A.M.I.E. Elec. Design 1993]

20. In a dc machine if the number of slots in the armature are more

(a) cooling is likely to be poor.
(b) commutation will be poor.
(c) cost will be more.
(d) the flux pulsation will be more. [A.M.I.E. Elec. Design 1994]